The present invention relates generally to apparatus for providing a measure of the distance between the apparatus and an object. In particular, the present invention is directed to automatic focusing systems in which a primary optical means, such as the taking lens of a camera, is moved to maintain an image of the object in focus at the plane of a photographic film.
One highly advantageous type of automatic focus apparatus is the spatial image correlation type. Examples of the different forms of arrangements of this type can be found in co-pending U.S. Patent applications Ser. No. 627,607, filed Oct. 31, 1975 now U.S. Pat. No. 4,002,899, issued Jan. 11, 1977, and Ser. No. 700,963 filed June 29, 1976 by Norman L. Stauffer, which are assigned to the same assignee as the present application, in U.S. Pat. Nos. 3,836,772, 3,838,275, and 3,938,117 by Norman L. Stauffer, and in U.S. Pat. No. 3,274,914 by Biedermann et al.
The typical spatial image correlation apparatus includes two auxiliary optical elements (for example, lenses or mirrors) and two detector arrays. The object distance is determined by moving one of the auxiliary optical elements relative to one of the radiation responsive detector arrays until they occupy a critical or correlation position. This position is a measure of the existing object to apparatus distance.
The relative movement of the auxiliary optical element and the detector array occurs for each distance measuring or focusing operation. The critical condition occurs when there is best correspondence between the radiation distributions of the two auxiliary or detection images formed on the two detector arrays. This condition of best distribution correspondence results in a unique value or effect in the processed electrical output signals. Typically, the correlation signal will contain a major extremum (either a peak or a valley) and one or more minor extrema. The major extremum is indicative of the distance to the object.
In most systems, the relative movement of the auxiliary optical element with respect to the detector arrays is achieved by moving a lens or mirror relative to one of the detector arrays. The particular position of the element when best distribution correspondence occurs provides a determination of the existing object to apparatus distance. The position of the auxiliary optical element at the time of best correspondence is used to control the position of the primary optical element, such as a camera taking lens.
In the previously mentioned co-pending application Ser. No. 700,963 by Norman L. Stauffer, the correlation signal includes a major peak which is indicative of the distance to an object. A peak detector is used to determine this major peak. A complete scan of all focus zones is provided to insure that the highest correlation is achieved. The location of the last and, therefore, highest peak detected corresponds to the desired focus position.
Continuous focus operation as required in a movie camera is achieved by determining the relative position of the lens and scanning mirror when the major peak is detected. Position correction signals are generated which cause the lens to be driven in either the near or the far direction. The system is constantly "hunting" for the correct focus position, since only the direction of focus error, and not its magnitude, is known. This oscillation can be eliminated by creating a region in time following the major peak or the lens-mirror relative position signal in which the occurrence of the other signal will inhibit motor operation (deadband). The use of a dead band is described in detail in the previously mentioned copending application by Dennis J. Wilwerding. In this type of system, it is desirable to have the deadband region small to minimize focus errors.